In this video you will learn about the process of cracking, how it is carried out and why
it is done.
From our video on fractional distillation, you should know that crude oil is a mixture
of hydrocarbons
with different carbon chain lengths. Smaller hydrocarbons have a lower boiling point and
are more
volatile, making them easy to ignite and more useful as fuels. Large hydrocarbons have a
higher
boiling point and are harder to ignite – they are not as useful as fuels.
We use a lot of fuels in our day to day lives, driving cars, aeroplanes, Lorries and even
heating our
homes. This means there is a very high demand for small chain hydrocarbons as they can be
better
fuels. However, after fractional distillation, we find that we have a supply of long chain
molecules.
The process of cracking breaks down long chain alkanes into a smaller alkane and alkene molecule.
This allows the supply of short chain hydrocarbons to meet the demand for them as the longer
chain
hydrocarbons are broken down into shorter alkanes and an alkene. The alkenes are also
useful as
these can be used in the manufacture of plastics.
Here is an equation showing that C12H26 is broken down into octane and butene.
There isn`t a set way that these molecules are broken down, in fact, C12H26 could also
be broken
down into decane and ethane or even butane, pentane and propene as these equations shows
(SHOW EQUATIONS)! The large hydrocarbon is broken down in quite a random way, just note
that
in each equation of the course, since we had 12 carbon atoms and 26 hydrogens going into
the
reaction, the same must come out!
The process of cracking is carried out in one of two ways, either by thermal cracking
using high
temperature and pressure or by catalytic cracking, at lower temperature and pressure but using
a
catalyst.
In thermal cracking the large hydrocarbons are heated to temperatures up to 750 deg C
and a high
pressure of 70 atmospheres. This high temperature and pressure breaks the bonds between carbon
atoms to make smaller molecules.
Catalytic cracking is the method used mostly in industry today. The large hardocarbons
are heated
to around 500 deg C and passed over a zeolite catalyst. The catalysts chosen are tend to
break the
bonds in the large hydrocarbon and making molecules of between 5-10 carbons in chain
length –
making them very useful in petrol.
Now at the end of this video tutorial, you should know that cracking is the process in
which large
alkanes are turned into smaller alkane and alkene molecules and that there are two types
of
cracking, thermal cracking using high temperature and pressure and catalytic cracking which
is
the method preferred in industry as it uses lower temperature and pressure and produces
more
molecules that are useful in petrol.